Natural gas and methane, a major constituent of natural gas, are difficult to economically transport and are not easily converted into liquid fuels or chemicals, such as gasoline, methanol, formaldehyde and olefins, that are more readily contained and transported. To facilitate transport, methane is typically converted to synthesis gas (syngas) which is an intermediate in the conversion of methane to liquid fuels, methanol or other chemicals. Syngas is a mixture of hydrogen and carbon monoxide with an H.sub.2 /CO molar ratio of from about 0.6 to about 6.
One chemical reaction effective to convert methane to syngas is steam reforming. Methane is reacted with steam and endothermically converted to a mixture of hydrogen and carbon monoxide. The heat energy to sustain the endothermic reaction is generated by the external combustion of fuel. The steam reforming reaction is of the form: EQU CH.sub.4 +H.sub.2 O.fwdarw.3H.sub.2 +CO
and produces syngas at an H.sub.2 /CO molar ratio of 3.
A second chemical reaction effective to convert methane to syngas is partial oxidation. Methane is reacted with oxygen in an exothermic reaction of the form: EQU CH.sub.4 +1/2O.sub.2.fwdarw.2H.sub.2 +CO. (2)
and produces syngas at an H.sub.2 /CO molar ratio of 2.
U.S. Pat. No. 5,306,411 to Mazanec, et al., that is incorporated by reference in its entirety herein, discloses the production of syngas by combined partial oxidation and steam reforming. The syngas is then converted to liquids by the Fischer-Tropsch process or can be converted to methanol by commercial processes.
In accordance with the Mazanec et al. patent, oxygen for an anode side reaction is obtained by contacting an oxygen-containing gas, preferably air, with the cathode side of a mixed conductor oxygen-selective ion transport membrane element and permeating oxygen by ion transport to the anode side of the mixed conductor. The membrane element has infinite oxygen selectivity. "Oxygen selectivity" is intended to convey that oxygen ions are preferentially transported across the membrane over other elements, and ions thereof. The membrane element is made from an inorganic oxide, typified by calcium- or yttrium-stabilized zirconia or analogous oxides having a fluorite or perovskite structure.
At elevated temperatures, generally in excess of 400.degree. C., the membrane element contains mobile oxygen ion vacancies that provide conduction sites for the selective transport of oxygen ions through the membrane elements. The transport through the membrane elements is driven by the ratio of partial pressure of oxygen (P.sub.o2) across the membrane: O-ions flow from the side with high P.sub.o2 to the side with low P.sub.o2.
Ionization of o.sub.2 to O.sup.- takes place at the cathode side of the membrane element and the ions are then transported across the membrane element. The O.sup.- ions then either combine to form oxygen molecules or react with a fuel, in either instance releasing e.sup.- electrons. Membrane elements that exhibit only ionic conductivity include external electrodes located on the surfaces of the membrane element. The electron current is returned to the cathode by an external circuit. Membrane elements having both ionic conductivity and electron conductivity transport electrons back to the cathode side internally, thus completing a circuit and obviating the need for external electrodes.
Commonly owned U.S. patent application Ser. No. 09/089,372 discloses the production of a product gas, typified by syngas, utilizing an oxygen selective ion transport membrane element to provide oxygen for combined endothermic and exothermic reactions where the overall reaction is exothermic or energy neutral. At least one of the endothermic reaction, the exothermic reaction and the internal heat transfer within the reactor is controlled to maintain the oxygen selective ion transport membrane within prescribed thermal limits since the membrane material will degrade at temperatures above about 1100.degree. C.
The ion transport membrane enables the local transfer of oxygen into the reaction passage to sustain the partial oxidation reaction without contaminating the reaction products with nitrogen. The balance between the reforming and partial oxidation reactions will depend on relative reaction kinetics which are influenced by the process feed composition, catalyst activity and the amount of oxygen transferred. The reactions typically are conducted at a temperature from 400.degree. C. to 1200.degree. C. and preferably between 800.degree. C. and 1050.degree. C. Since the partial oxidation reaction is exothermic and the reforming reaction endothermic, the balance between the two will determine whether the overall process is exothermic or endothermic. Depending on the operating pressure the process is energy neutral at H.sub.2 /CO molar ratios in the range of 2.3 to 2.5, produces excess energy below that range and requires additional heat above the range.
In accordance with the Ser. No. 09/089,372 patent application, the heat generated by the exothermic partial oxidation reaction is sufficient to satisfy the requirements of the endothermic reaction and, preferably, generates a heat surplus to compensate for thermal losses.
When the exothermic reaction is partial oxidation of methane, the reaction generates two moles of hydrogen for every mole of carbon monoxide produced. When the endothermic reaction is steam reforming, the reaction generates three moles of hydrogen for every mole of carbon monoxide produced.
The process and reactor designs disclosed in the Ser. No. 09/089,372 application are particularly suited for generating syngas with H.sub.2 /CO molar ratios in the range of 2.3 to 2.5, dependent on reactor pressure.
For certain chemical processes, it is desirable to have syngas with an H.sub.2 /CO molar ratio greater than about 2.3.
To shift the H.sub.2 /CO ratio to greater than 2.3 to 2.5, it is possible to generate additional heat by driving the partial oxidation reaction towards more complete oxidation. This approach also generates more H.sub.2 O and more CO.sub.2 that must be removed from the product gas at some expense. In addition, the additional fuel burned during oxidation is high grade, and therefore expensive, natural gas.
A second approach is to provide externally generated heat to the reactor. This approach is also less than satisfactory because of the associated cost.
U.S. Pat. Nos. 5,565,009 and 5,567,398 to Ruhl, et al., that are incorporated by reference in their entireties herein, disclose manufacturing syngas by steam reforming of methane in a catalyst bed located on the shell side of a tube and shell reactor. The heat for sustaining the reforming reaction is provided by combustion of fuel within tubes where the fuel and oxygen supply (air) are separately heated and only combined after they reach their auto-ignition temperature. The oxygen is provided by air and the nitrogen contained within that air is heated during combustion to form a number of detrimental NOx compounds that can only be removed from the combustion products gas with difficulty.
U.S. patent application Ser. No. 08/848,204 entitled "Solid Electrolyte Ion Conductor Reactor Design" by Gottzmann, et al., that was filed on Apr. 29, 1997, now U.S. Pat. No. 5,820,655 and it is incorporated by reference in its entirety herein, discloses using the heat generated by an exothermic oxidation reaction to heat an oxygen-containing feed gas prior to delivery of that feed gas to the cathode side of an oxygen-selective ion transport membrane element. The Ser. No. 08/848,204 application also discloses the use of a thermally conductive shroud tube surrounding the membrane elements to enhance the transfer of heat while maintaining isolation of gases.
While the aforementioned disclosures recite processes and reactors for the production of syngas utilizing an oxygen-selective ion transport membrane element and utilizing the heat generated by an exothermic partial oxidation reaction to drive an endothermic steam reforming reaction, they are generally limited to the production of syngas with H.sub.2 /CO molar ratios of from 2.3 to 2.5, depending on reaction side pressure, and where the heat released by the exothermic partial oxidation reaction is equal to or greater than the heat required for the endothermic reforming reaction. Higher molar ratios are obtainable by providing additional heat to drive the steam reforming reaction, but this approach requires the addition of externally generated heat, at a significant expense, and is typically associated with the formation of undesirable NOx compounds.
There remains, therefore, a need for a method to generate syngas having H.sub.2 /CO molar ratios higher than 2.3 to 2.5 that does not have the limitations of the prior art.